(1) Field of the Invention
This invention relates to a semiconductor device and, more particularly, to a semiconductor device which realizes a reduction in consumption of power by periodically turning internal power on and off in a stanby state at nonuse time like large-scale integration (LSI) in portable remote terminal units used in a direct-sequence spread spectrum communication system.
In recent years microfabrication technologies have been adopted in the process for producing LSI and wiring width is approaching 0.10 μm. These microfabrication technologies increase the packaging density of transistors, but threshold voltages LSI needs to operate must be lowered. This causes the substantial problem of a leakage (quiescent) current between transistors increasing at standby time. A portable remote terminal unit, such as a cellular telephone, which is driven by a battery is always in a standby state even at nonuse time. Therefore, an increase in this leakage current will shorten nonstop telephone call time and standby time. This is a very serious problem from a practical viewpoint.
(2) Description of the Related Art
Conventionally, the power has been disconnected from sections which do not need to operate in a standby state to prevent consumption of power from increasing due to such a leakage current. In addition, in LSI, the power has been disconnected from blocks which do not need to operate to prevent consumption of power from increasing due to such a leakage current.
When the power is disconnected from what is called processors, such as a digital signal processor (DSP) and central processing unit (CPU), included in LSI, all the data the processors were processing until then will be lost. It therefore is necessary to save it in a nonvolatile memory for the reapplication of the power. That is to say, the state before the power being turned off is saved in a memory outside the LSI where the power is not disconnected. When power is reapplied, data saved in this memory will be restored.
The boot process of loading a program from an external memory is performed by a program in a processor each time the power is turned on.
FIG. 10 is a view for describing the timing with which a boot process is performed.
Each time a boot process is performed, an error detection process called checksum is performed after a program being loaded from an external memory to judge whether the program was read normally.
For example, with a cellular telephone in a standby state, power is applied only during intermittent receiving to reduce consumption of power. That is to say, the power is disconnected when intermittent receiving is not performed. At the time when a boot is performed for this intermittent receiving, error detection is performed.
This intermittent receiving is the operation of checking whether the owner of a cellular telephone had a telephone call from a base station. A cellular telephone is put into a receiving state in a cycle between, for example, one and two seconds. If there is no incoming call, then the power is disconnected.
FIG. 11 is a view for describing an intermittent receiving cycle and checksum time.
With a cellular telephone, a boot interval is the same as an intermittent receiving cycle in a standby state and is usually 1.28 or 2.56 seconds. If a cellular telephone is in a standby state, power is applied once every intermittent receiving cycle. This intermittent receiving cycle includes program load time and checksum execution time.
With a cellular telephone and the like which can be in a standby state for hundreds of hours, however, the percentage of accumulated time for which checksum is performed to the total time for which the power should be in the ON state will be high if the checksum is performed at the above boot intervals. A system cannot operate while checksum is being performed, so there are many blocks where a useless leakage current flows. This means that a battery is used wastefully. Moreover, a program is loaded every time, so a useless leakage current also flows during load time.